For in-application representation of the 'shine' on an icon, you can create a custom UIView that draws the shine gradient, using the code here (adjusting the gradient colors to match Apple's). When you want to apply the 'shine' to the preview icon, just overlay this custom UIView on top of its UIImageView (or whatever you're hosting it in).

The rectangular clipping could be a little trickier. If you have a solid black background, you could overlay a frame UIImageView that has a black area with a rounded rectangular transparent region in its center. You can also do this in a more general-purpose manner via Core Graphics by drawing your image and the gloss into a view, then using CGContextClip with a rounded rectangle to carve out the rounded interior.